germanium(Ge), a chemical element between silicon and tin in Group IVa of the periodic table, a silvery-gray metalloid, intermediate in properties between the metals and the nonmetals. Although germanium was not discovered until 1886 by Clemens Winkler, a German chemist, its existence, properties, and position in the periodic system had been predicted in 1871 by the Russian chemist Dmitry Ivanovich Mendeleyev, who called the hypothetical element ekasilicon. Germanium did not become economically significant until after 1945, when its properties as a semiconductor were recognized as being of value in electronics. Many other substances now also are used as semiconductors, but germanium remains of primary importance in the manufacture of transistors and of components for devices such as rectifiers and photocells.

A rare element never found free in nature, germanium occurs mainly as a constituent of such uncommon minerals as argyrodite (from which it was first isolated), germanite, and renierite. The occurrence abundance of germanium in the Earth’s crust is low—about 1.5 parts per million. In refining germanium, the low-grade residues obtained from its ores are treated with strong hydrochloric acid, and the resulting germanium tetrachloride is distilled, purified by repeated redistillation, and hydrolyzed to form germanium dioxide, which is then reduced by hydrogen to a powdery form of the metal that is melted at a temperature of about 1,100° C (2,000° F [in an inert atmosphere]) and cast into ingots or billets.

The element is brittle rather than ductile; the atoms in its crystals are arranged as are the carbon atoms in diamond. It is not attacked by air at room temperature but is oxidized at 600°–700° C (1,100°–1,300° F) and reacts quickly with the halogens to form tetrahalides. Among the acids, only concentrated nitric or sulfuric acid or aqua regia (a mixture of nitric and hydrochloric acids) attack germanium appreciably. Although aqueous caustic solutions produce little effect on it, germanium dissolves rapidly in molten sodium hydroxide or potassium hydroxide, thereby forming the respective germanates.

Germanium forms stable oxidation states of +2 and +4, the compounds of the latter being more stable and numerous. The two most important compounds of germanium are the dioxide (GeO2) and the tetrachloride (GeCl4). Germanates, formed by heating the dioxide with basic oxides, include zinc germanate (Zn2GeO4), used as a phosphor (a substance that emits light when energized by radiation). The tetrachloride, already mentioned as an intermediate in obtaining germanium from its natural sources, is a volatile, colourless liquid that freezes at about -50° C (-58° F) and boils at 84° C (183.2° F).

For use in electronic devices, germanium ingots or billets require further purification, which usually is effected by the technique of zone refining. The highly pure germanium is then melted and “doped” to produce desired electronic characteristics by adding by the addition of minute amounts of arsenic, gallium, or other elements to produce desired electronic characteristics. Finally, single crystals are generated from the melt at carefully controlled temperatures, using a seed crystal as a nucleus.

In addition to its applications in electronic devices, germanium is used as a component of alloys and in phosphors for fluorescent lamps. Because germanium is transparent to infrared radiation, it is useful employed in equipment used for detecting and measuring such radiation, such as windows and lenses. The high index of refraction of germanium dioxide renders it valuable as a component of glasses used in optical devices, such as wide-angle lenses for cameras and microscope objectives.